color stop positions

I'm not used to how PostCSS works under the hood so this might be a silly question.

Is it possible this plugin could support a subset of the positional arguments for color stops eg:

linear-gradient(
  to top,
  black 0%,
  ease-in-out,
  transparent 15%,
  linear,
  transparent 100%
);

I understand that this probably not possible to polyfill the entire set of positional arguments (calc, mixing %/px etc.) but surely just linearly interpolating between percentages should be possible given the output css uses percentages?

I came across this limitation while using mask-image with the linear gradient as a fade-out on the bottom of an image. This problem would usually be easily alleviated with background-size but, unfortunately, mask-image-size works differently such in such a way that anywhere not covered by mask-image is masked out. So the image fades nicely then just becomes transparent for the most part.

The issue names no files or tests. Start by locating the PostCSS gradient-parsing entry point and how easing gradients are currently converted; define the supported positional color-stop subset and verify that the example produces percentage-based output suitable for mask-image.